Why Is This Year 5775 on the Jewish Calendar?
This year (September 25, 2014 - September 13, 2015) is 5775 on the Hebrew calendar because the Jews count their years from 3761/3760 BC. That is the year of the commencement of the lunisolar calendar, not the year of Creation.
In the century following the fall of Jerusalem (AD 70), certain Jewish scholars took the date 3761/3760 BC and assigned it to the time described in Genesis 1 and 2. Thus, the Jews preserved the correct date for calendar calculation but mistakenly associated it with Creation week.
According to the best estimates, Creation occurred over two centuries prior to the beginning of the lunisolar calendar. The basic factor upon which the lunisolar calendar is based, the average length of the lunar month, would have taken more than two centuries of measurement and observation to determine.
The Jews celebrate their new year in the autumn on the first day of Tishri, the seventh month. The Bible refers to two different years, the civil and the sacred. In ancient Israel, the civil year began in the autumn, on Tishri 1. It is comparable to the fiscal year in the United States. It was more or less the legal year, which was used to reckon the reign of kings and other chronological events.
The true sacred year of the Bible begins with the first day of the month of Nisan (also called Abib), in the spring (Exodus 12:2). The sacred year portrays God's great plan through the progression of holy days from Passover to the Last Great Day.